title: UV photodissociation of pyrroles: symmetry and substituent effects.
abstract: H (Rydberg) atom photofragment translational spectroscopy and ab initio electronic structure calculations are used to explore ways in which ring substituents affect the photofragmentation dynamics of gas phase pyrroles. S1 ← S0 (σ* ← π) excitation in bare pyrrole is electric dipole forbidden but gains transition probability by vibronic mixing with higher electronic states. The S1 state is dissociative with respect to N-H bond extension, and the resulting pyrrolyl radicals are formed in a limited number of (nontotally symmetric) vibrational levels (Cronin et al. Phys. Chem. Chem. Phys. 2004, 6, 5031-5041). Introducing σ-perturbing groups (e.g., an ethyl group in the 2-position or methyl groups in the 2- and 4-positions) lowers the molecular symmetry (to C(s)), renders the S1-S0 transition (weakly) allowed, and causes some reduction in N-H bond strength; the radical products are again formed in a select subset of the many possible vibrational levels but all involve in-plane (a') ring-breathing motions as expected (by Franck-Condon arguments) given the changes in equilibrium geometry upon σ* ← π excitation. The effects of π-perturbers are explored computationally only. Relative to bare pyrrole, introducing an electron donating group like methoxy (at the 3- or, particularly, the 2-position) is calculated to cause a ∼10% reduction in N-H bond strength, while CN substitution (in either position) is predicted to cause a substantial (∼3000 cm(-1)) increase in the S1-S0 energy separation but only a modest (∼2%) increase in N-H bond strength.
authors.affiliation: School of Chemistry, University of Bristol , Cantock's Close, Bristol, BS8 1TS, U.K.
authors.forename: Tolga N V
authors.initials: TN
authors.lastname: Karsili
authors.forename: Barbara
authors.initials: B
authors.lastname: Marchetti
authors.forename: Roberta
authors.initials: R
authors.lastname: Moca
authors.forename: Michael N R
authors.initials: MN
authors.lastname: Ashfold